This invention relates to a bi-directional horn for ultrasonically consolidating a fiber reinforced composite structure.
Composite materials are becoming more and more attractive for a wide variety of uses, from aircraft and automobiles to sporting goods and toys, because of their high stiffness and strength-to-weight ratio. A composite material is a combination of fibers in a matrix or resin. Typically, a composite structure is made of a number of layers or plies of such composite materials. Typically, composite materials include a combination of fibers or fibrous tows in a matrix of thermoplastic or thermosetting resin. Dry fabric with unidirectional fibers or fibrous tows or woven fibers is often precombined with resin as a xe2x80x9cPREPREGxe2x80x9d. Examples include carbon, glass or graphite fibers in a resin matrix. The fibers typically comprise more than 35% of the material volume. Dry fabric with unidirectional fibers or fibrous tows or woven fibers can also be precombined with at thermosetting resin. This type of composite, a thermoset composite, generally requires that the fiber/resin plies be laid-up, debulked, and then curedxe2x80x94a process which can take a matter of hours. These composites are contrasted with thermoplastic composites which are generally faster to fabricate because there is no curing involved. The thermoplastic resin infused plies need only be heated to melt the plastic matrix and then pressed together or consolidated to the other plies before cooling. With thermosetting composites, on the other hand, heating to a high enough temperature invokes an exothermic reaction causing the molecules of the resin to cross link. Once this chemical cross linking occurs, the viscosity of the resin cannot be lowered. This is not the case with thermoplastic resins.
As used herein, consolidation means laminating two or more plies together to form a part or structure. Good consolidation implies a low level of voids (typically less than 3%) and a shear strength of the ply interfaces after curing which approaches that of the resin matrix.
Heating the plies, to consolidate them however, is troublesome: a number of different heating techniques have been tried but have met with mixed success. Laser heating in the nip between the previous ply and the ply being applied or laid down has not been wholly successful due to the practical problems of applying the energy at the nip. A laser apparatus with all the necessary controls is also quite expensive. Infrared devices, which depend upon radiant heating, suffer from poor heat moduability and can easily damage the composite structure.
Microwave devices suffer similar shortcomings and are potentially hazardous to surrounding personnel as well. A hot shoe technique, which is commercially available, employs a series of massive, heated, iron-like devices. This method relies on conduction through the ply to heat the interface which makes the process a slow one. Because of size and geometry, this method has only been applied to the production of flat panels, thereby restricting its usefulness.
The most evaluated technique presently in use is hot gas heating. In that process, a stream of hot air or gas is aimed into the nip between the new ply (layer or tape or tow) and the substrate and thereafter the new ply is pressed or ironed onto the substrate using a pinch roller or a shoe. While the consolidation levels achieved using this approach are high, the heating is difficult to modulate with respect to rapid changes in the material feed rate. This complicates the practical integration of convective, hot gas heating with standard computer-numerical-control fabrication equipment. Moreover, despite the high consolidation, some reports on the mechanical properties of the resulting composites have been disappointing. This may be due to damage or degrading of the surface of the material at the nip, especially due to the high heat level applied and the large temperature differential (300xc2x0 C. or more) between the hot gas stream and the melt temperature of the thermoplastic material.
Filament winding, tape placement and tow placement are also common methods for fabricating parts from fiber reinforced composites.
Filament winding involves winding a filament bundle known as a xe2x80x98towxe2x80x99, to which resin has previously been applied, around a mandrel. Multiple turns around the mandrel are used to build up the required part thickness after which the part is cured in an oven or autoclave.
During winding, thicker parts may require intermediate consolidation or compaction steps known as xe2x80x98debulksxe2x80x99 using heat in conjunction with pressure and/or a vacuum. Thick parts cured without any intermediate debulks often develop fiber wrinkling which degrades the mechanical properties of the cured part.
In tape or tow placement, a robotic head is used to place a narrow prepreg tow or tape (typically 0.125-2 inches in width) against a tool which defines the desired part shape. Multiple layers are placed at different orientations to obtain the required ply construction and part thickness. A combination of downward pressure on the tow, applied by the head, and tack (stickiness of the tow) is required to insure the tow remains in location after placement, particularly when placing a tow on concave portions of the tool.
Usually the tow, and the previously deposited ply layers, are heated to increase the tack prior to placement by the robotic head.
Current tow placement machines use separate mechanisms, placed in close proximity, to apply heat and pressure. Commonly, heat is applied by a jet of hot gas directed onto the tow and pressure is applied by one or more rollers or shoes which ride against the surface of the tow. The levels of consolidation achieved in this manner are such that thick tow or tape placed parts also require intermediate debulking to prevent fiber movement or wrinkling during cure.
One obstacle to consistently achieving higher levels of consolidation with these processes is the difficulty inherent in controlling temperature. Because of the heat capacity present in a hot gas system, the temperature of the gas jet, and hence the heat input to the tow cannot be easily modulated to allow for starts, stops or changes in advance rate of the robotic head.
Intermediate debulking typically involved applying a vacuum bag along with associated bag sealants, vacuum lines, connections, etc. to the layup tool or mandrel, and transfer of the tool from the tow placement machine to an oven or autoclave where it is heated to 180-250xc2x0 F. and held under vacuum pressure for up to four hours. The part is then returned to the tape placement machine to continue the lay-up process. Current thick parts such as the V-22 spindle and the F-22 pivot shaft require numerous intermediate debulks which adds substantial cost.
A method of applying heat and pressure which achieves high levels of consolidation during tape or tow placement, thus eliminating the need for intermediate debulks, is desired and could result in substantial cost savings. The current invention relates to such method which uses a bi-directional device to generate the heat and pressure required for consolidation. Further, the method has the potential, in certain cases, to replace autoclave curing with curing in an oven. Moreover, the unique design of the bi-directional device of this invention allows the ultrasonic horn to be driven over the laminate bi-directionally while remaining in constant contact to the top surface of the composite.
A bi-directional device utilizing ultrasonic energy to heat the plies is appealing for a number of reasons. Unlike convection (hot gas), conduction (hot shoes/irons), or radiation (infrared), ultrasonic consolidation does not depend upon a thermal driver to effect energy transfer to the composite material. Ultrasonic heating is instantaneously modulatable, and it provides deep, penetrating heating in the polymeric matrix beyond mere surface heating.
Ultrasonic welding has long been used to weld or bond neat (unreinforced) plastics with no or little fiber content. Such welding is done by placing an ultrasonic horn perpendicular to two plastic layers, pressing down on the layers and energizing the horn. Obeda, U.S. Pat. No. 4,713,131, teaches joining large sheets of polypropylene plastic by overlapping the sheets of plastic and welding their edges together using an ultrasonic horn placed between the sheets. Obeda, however, teaches nothing about composite materials.
But, others have attempted to use an ultrasonic horn to fabricate composite parts. See Joining Methods for Plastic and Plastic Composites: An Overview, Vijay Stokes, Polymer Engineering and Science, Mid-October 1989, Vol. 29, No. 19, p. 1310-1324, specifically pp. 1322-1324, items 168-236. These previous attempts to weld thermoplastic composites during the lamination process used a conventional ultrasonic horn with a flat face disposed perpendicularly with respect to the plies. These techniques have yielded disappointing results because, it is speculated, the presence of the fibers alters the energy transfer in the material. Moreover, these conventional ultrasonic welding techniques set up a compression wavefront in the material which does not transmit well through the material. In 1987, engineers at Martin Marietta attempted to use an ultrasonic horn to consolidate composite resin-fiber plies. The horn was placed on the top of two moving plies to be consolidated in a direction perpendicular to the plies. A range of different pressures, energy levels, and feed rates were tried. The result, however, was not satisfactory: xe2x80x9cC-Scan results have shown that attempts to produce consolidated or near-consolidated laminates have not been successful thus far . . . xe2x80x9cSonic Assisted Process Developmentxe2x80x9d, Interim Technical Report, xe2x80x9d contract No. F 33615-86-5041, Martin Marietta Baltimore for Material Laboratory Air Force Wright Labs., March 1987.xe2x80x9d
Therefore, although ultrasonic horns have been used to weld plastic sheets together and, to some extent, have been successfully used to weld thermoplastics containing up to about 35% filler (such as glass or talc), the state of the art reveals no successful methodology of fabricating fiber reinforced composite structures using an ultrasonic horn to consolidate and further debulk the individual fiber-resin plies.
The applicants hereof then invented a solution: when a conventional flat faced ultrasonic horn was angled and moved relative to the plies, the plies were more fully consolidated. See the application filed by the instant inventive entity on Oct. 11, 2000 which is a continuation of application Ser. No. 08/394,737 filed Feb. 25, 1995 and the application filed by the instant inventive entity on Oct. 19, 2000 which is a continuation-in-part of application Ser. No. 08/394,737 filed Feb. 25, 1995 incorporated herein by this reference.
One potential problem however, is that conventional horns have a square flat face which is difficult to push across the surface of the top most ply. Most conventional horns have a tapered tip leading to flat square face which creates a distinct sharp edge. When the horn is angled to provide consolidation of a fiber reinforced composite structure, the sharp edge of the horn may dig in and gouge the composite structure.
It is therefore an object of this invention to provide an improved ultrasonic horn for ultrasonically consolidating a fiber reinforced composite structure.
It is a further object of this invention to provide such a horn which can be used in a bi-directional fashion.
It is a further object of this invention to provide such a horn for ultrasonically consolidating a fiber reinforced composite structure which provides a constant contact area over a range of angles between the horn and the plies.
It is a further object of this invention to provide such a horn for ultrasonically consolidating a fiber reinforced composite structure which provides for faster and more efficient consolidation.
It is a further object of this invention to provide such a horn for ultrasonically consolidating a fiber reinforced composite structure which provides a constant contact surface area between the device and the fiber reinforced composite structure.
It is a further object of this invention to provide such a horn for ultrasonically consolidating a fiber reinforced composite structure which reduces hot spots in the horn and the processed material.
It is a further object of this invention to provide such a horn for ultrasonically consolidating a fiber reinforced composite structure which produces a more even heat distribution on the fiber reinforced composite structure.
It is a further object of this invention to provide a method of fabricating a fiber reinforced composite structure which utilizes an improved ultrasonic horn to consolidate the fiber-resin plies of the composite structure.
It is a further object of this invention to provide such a method which utilizes an improved ultrasonic horn and is controllable, instantly modulatable, and which does not require a large thermal differential between the device and the material.
It is a further object of this invention to provide such a method which is much less likely to cause overheating or damage to the material or detract from the consolidation quality.
It is a further object of this invention to provide such a method which applies pressure simultaneously with heat.
It is a further object of this invention to provide such a method which is faster and easier to employ and is less expensive both in execution and in the equipment required, and is extremely energy-efficient.
It is a further object of this invention to provide such a method which eliminates the repeated debulking operations required of the prior art and in which debulking occurs as the plies are laid down.
It is further an object of this invention to provide such a method in which the ultrasonic horn can be used not only to debulk the plies as they are laid down, but which can also advance the chemical reaction of the resin so that it approaches a condition commonly referred to as the xe2x80x98gelation pointxe2x80x99 thus making it possible to effect a final cure in an oven instead of in an autoclave.
This invention results from the realization that a truly effective and robust bi-directional device for ultrasonically consolidating a fiber reinforced composite structure can be achieved by providing an ultrasonic horn which includes a tip at the distal end of the horn which terminates in a rounded face that allows the horn to be angled relative to the composite structure. The unique rounded face of the horn contacts the top ply of the fiber reinforced composite structure enabling the horn to be both driven and pulled over the fiber reinforced composite structure.
This invention features a bi-directional device for ultrasonically consolidating a fiber reinforced composite structure including a horn which can be angled relative to the composite structure. The horn also includes a tip at the distal end of the horn, the tip terminating in a rounded face which contacts the composite structure enabling the horn to be driven bi-directionally over the composite structure at an angle.
The bi-directional device in accordance with this invention may include a horn which is a step horn, an exponential horn, a catenoidal horn, or a rectangular horn.
The tip of the horn may include sides which converge toward the rounded face at an angle of between 10xc2x0 and 40xc2x0. The distal end of the tip may be symmetrical, but alternatively, the horn may include a non-rounded portion proximate the face on the distal end of the tip.
This invention also features a method of fabricating a fiber reinforced composite structure. The method includes the steps of assembling a stack of fiber reinforced material plies, engaging an ultrasonic horn including a rounded face with a top surface of the upper most ply such that the rounded face contacts the upper most ply, orienting the horn at an acute angle with respect to the top surface and energizing the horn to induce a shear wave in the plies to heat the plies. A relative motion is provided between the horn and the stack such that the horn can be driven and pulled over the upper most ply to consolidate the plies. The method may further include the step of applying a consolidation force through the horn or proximate the horn by a roller. Preferably, the acute angle of the horn with respect to the top surface is less than or equal to 45 degrees.
The fiber matrix structure may be a thermoplastic polymer-matrix or a thermosetting polymer-matrix.
This invention further features a method of fabricating a fiber reinforced composite structure which includes the steps of assembling a stack of fiber reinforced material plies, engaging an ultrasonic horn with a tip at the distal end of the horn terminating in a rounded face which contacts a top surface of the upper most ply and enables the horn to be driven bi-directionally over the top surface of the upper most ply, orienting the horn at an acute angle with respect to the top surface, and energizing the horn to induce a shear wave in the plies to heat the plies. The horn is driven bi-directionally over the top surface to consolidate the plies.
This invention further features a method of fabricating a thermosetting matrix fiber reinforced composite structure. The method comprises assembling a stack of fiber reinforced thermosetting resin material plies, engaging an ultrasonic horn with a tip at the distal end of the horn terminating in a rounded face which contacts a top surface of the upper most ply and enables the horn to be driven bi-directionally over the top surface of the upper most ply, orienting the horn at an acute angle with respect to the top surface, and energizing the horn to induce a shear wave in the plies to heat the plies. A relative motion is provided between the horn and the stack such that the horn is driven and pulled over the upper most ply to consolidate the plies. The energy level applied by the horn is sufficient to reduce the viscosity of the thermosetting resin to the point where the plies can be debulked but not high enough to fully cross-link the resin so that another ply can be cross-linked to the uppermost ply. Pressure is applied to the plies via the horn and/or a separate roller or shoe to debulk the plies.
The plies of thermosetting material typically have more than 40% fiber by volume. Final curing may take place in an oven or an autoclave. One or more plies may be deposited onto a previously assembled and consolidate stack of plies. Preferably, the method of the subject invention may be used in conjunction with filament winding, tape placement, fiber placement or tow placement or tow placement to deposit and consolidate thermosetting matrix, fiber reinforced composites.